In a conventional scroll-type expansion machine, a compression chamber of compression means is defined by a first fixed scroll and an orbiting scroll on one hand, and an expansion chamber of expansion means is defined by a second fixed scroll and the orbiting scroll. The orbiting scroll is connected to a crank shaft for being driven in an orbiting motion by a motor mounted to the crank shaft while being supported not to make a spinning motion. Also, a discharge port of a compression mechanism and a suction port of an expansion mechanism respectively are directly connected to one end of pipes for connection to the heat exchanger, whereby the suction port of the compression mechanism and the discharge port of the expansion mechanism are defined in a passage remote from the support mechanism (see Japanese Patent Publication No. 07-037857).
Such an expansion machine has a structure in which an expansion mechanism for expanding the refrigerant and a sub-compression mechanism driven by a recovered power to participate into a part of compression process are accommodated within a hermetic vessel, the hermetic vessel having maintained therein lubricating oil for the sliding portions. In the refrigeration cycle employing such the expansion machine, the lubricating oil is held at two locations of the main compressor and the expansion machine, so that the oil level must be controlled not to generate a shortage of the lubricating oil therein.
Therefore, in the refrigeration air conditioner employing the conventional expansion machine, the pressure within the hermetic vessel containing the expansion mechanism and the sub-compression mechanism is made equal or substantially equal to the discharge pressure of the main compressor, so that the expansion mechanism suctions the refrigerant from the upper portion of the expansion machine vessel, and the main compression machine is provided, when the atmosphere within the main compressor vessel is at the suction pressure, with a suction portion of the compressor above the oil level, and is provided, when the atmosphere within the main compressor vessel is at the discharge pressure, with a discharge port of the vessel above the oil level, so that the superfluous oil within the main compressor vessel can be returned together with the refrigerant to the expansion machine vessel through an external circuit, as disclosed in Japanese Patent Laid-Open No. 2004-325018.
In another refrigeration air conditioner, the pressure within the hermetic vessel containing the expansion mechanism and the sub-compression mechanism is made equal to the discharge pressure of the sub-compressor so that the expansion mechanism directly sucks the refrigerant from the outside of the expansion machine vessel and directly discharge the expanded refrigerant to the outside of the expansion vessel, and the main compressor is provided, when the atmosphere within the main compressor vessel is at the suction pressure, with a suction port of the compression mechanism above the oil level, and is provided, when the atmosphere within the main compressor vessel is at the discharge pressure, with a discharge port from the compression mechanism above the oil level, so that the superfluous oil within the main compressor vessel can be returned together with the refrigerant to the expansion machine vessel through an external circuit, as disclosed in Japanese Patent Laid-Open No. 2004-325019.